This invention relates to an envelope generator of an electronic musical instrument.
In a conventional envelope generator, a single envelope shape of a predetermined shape is stored in an envelope memory and this envelope shape is read out of the envelope memory by driving a readout control counter with a constant clock pulse. Accordingly, only one envelope shape can be produced by one such conventional envelope generator.
It is an object of the present invention to provide a novel envelope generator capable of selectively producing envelope shapes of different shapes with a single memory and counter. According to the invention, contents of a memory are so set that the amplitude level of an envelope shape will correspond to a count value of a memory readout control counter whereby an envelope shape whose amplitude varies in accordance with variation in the count value of the counter is produced. Further, various counting modes of the counter are set by a counter control circuit in accordance with various envelope shapes and a desired one of such envelope shapes is selectively produced by controlling the counting operation of the counter in accordance with a selected one of such counting modes. The term "counting modes" means modes of all elements concerning the counting operation of the counter including timings of starting, stopping and resuming counting, an order of upcounting and downcounting, a counting speed (i.e. a rate of a counting clock pulse) and a switching timing of the counting speed. A term "mode" hereinafter used signifies the above described "counting mode".
In the embodiment of the present invention to be described hereinbelow, four kinds of envelope shapes, i.e., "direct keying mode (A)", "sustain mode (B)", "percussive damp mode (C)" and "percussion mode (D)" as shown in FIG. 8 can be produced. An envelope generation control logic is provided for selectively controlling the counting operation of the counter with respect to each of the above described four kinds of modes. This selection is designated by an envelope mode select signal. The selected counting mode can be changed by a curve select signal.
In a case where the envelope shape of the percussive damp mode is generated by a conventional envelope generator of a type wherein an envelope shape is read out by using a counter and an envelope memory, the envelope abruptly falls upon release of a depressed key, as shown in FIG. 1(a). This abrupt fall of the envelope causes an ending of a musical tone resembling a click sound. The envelope of the percussive damp mode is generally used for simulating a sound which is produced when a depressed piano key is released during production of a tone and the sound resembling a click sound is undesirable for an ending of a piano tone. According to the present invention, the envelope in the percussive damp mode is provided with a decay shape which, as shown in FIG. 1(b), falls quickly upon release of the depressed key in response to a high rate damp clock pulse whereby generation of the undesirable click sound is prevented.